1. Field of the Invention
The present invention is directed to an X-ray system with an X-ray apparatus including an X-ray source and an X-ray receiver that is adjustable relative to a subject to be examined for registering 2D projections of a region of the subject, with subsequent 3D image reconstruction of the subject.
2. Description of the Prior Art
An X-ray system of the above type usually has a C-arm for carrying the X-ray source and the X-ray receiver, this C-arm being seated such in a holder at the X-ray system so that it is motor-adjustable in a specific angular range along its circumference (orbital motion). For acquiring 2D projections from different projection angles for 3D image reconstruction of, for example, a body region of a subject with the assistance of the C-arm X-ray system, the C-arm--after appropriate placement relative to the subject--is adjusted along its circumference during the registration of 2D projections of the body region of the subject. 3D images of the body region of the life form are subsequently reconstructed from the 2D projections registered with the X-ray system during the adjustment motion of the C-arm. The 3D reconstruction, however, assumes exact knowledge of the projection geometry, i.e. the knowledge of the position of the X-ray source and the X-ray receiver during each of the individual 2D projections.
Known stationary and particularly mobile C-arm X-ray systems exhibit mechanical instabilities particularly relating to the adjustment of the C-arm along its circumference, causing deviations of the real adjustment motion of the C-arm from the ideal adjustment motion to occur. The determination of the projection angles is thus often affected by errors, the quality of the 3D images reconstructed from the 2D projections suffering therefrom.
The following two methods are known for avoiding errors in the determination of the projection angles.
German OS 195 12 819 discloses the use of a marker ring usually formed of plexiglass with inserted metal structures, that is arranged around the body region of the subject to be examined. The metal structures of the marker ring are visible in the 2D projections of the body region to be examined, so that the respective projection angles of the 2D projections can be calculated from their position. This method, however, has the disadvantage that the marker ring has a relatively large diameter, so that the spacing between the X-ray source and the marker ring is very small (a few centimeters) given C-arm with a small diameter. The metal structures therefore appear extremely enlarged in the 2D projections, so that large parts of the 2D projections are overlaid by the metal structures. Further, only a small region of the metal structures of the marker ring is imaged in the 2D projections, so that the determination of the projection angles is difficult on the basis of the slight number of imaged metal structures.
Calibration measurements can be made before the actual patient measurement, assuming that the system behavior, i.e. essentially the adjustment motions of the C-arm, is reproducible to a high degree. This method, however, is extremely time-consuming and, moreover, can only be applied with a mechanically reinforced, stationary C-arm X-ray apparatus. Application to mobile X-ray apparatus is not possible because of the aforementioned mechanical instability of such an X-ray apparatus. Mechanical stabilization of a mobile X-ray apparatus is not possible because of the great increase in weight that limits the mobility.
U.S. Pat. No. 5,109,397 discloses a mobile computed tomography system having an X-ray system rotating around a rotation center and including an X-ray source and an X-ray receiver, to which sensors are allocated. These sensors move along with the X-ray system and interact with a stationary ring allocated to the rotation center for detecting lateral movements of the X-ray system during a scan. The sensors generate signals which are evaluated to allow the spacings between their defined points of attachment and the ring to be determined. The acquired data are subsequently utilized in the reconstruction of tomograms. The ring is thereby arranged in the propagation path of an X-ray beam emanating from the X-ray source.
Sensors for determining the distance of two objects from one another are also disclosed in German PS 43 32 254 and German Utility Model 94 08 562. The determination of the distance ensues by measuring the transit time of acoustic waves or electromagnetic waves.
German OS 36 04 955 discloses an X-ray diagnostic apparatus having an image generating system with an X-ray radiator and a radiation receiver as well as a patient table. Position sensors in the form of potentiometers that acquire the position of adjustable components of the image generating system are connected to these components.
German OS 195 35 583 also discloses an X-ray diagnostics apparatus with a positioning aid. A light transmitter for emitting a light beam is provided at the X-ray image intensifier so that this light beam is focused onto a X-ray radiator disposed opposite the X-ray image intensifier. In this way, a positioning of the X-ray radiator and the X-ray image intensifier can ensue with reference to an examination subject without emitting X-rays.